1. Technical Field
The invention relates generally to an improved auxiliary axle assembly for land vehicles. More particularly, the invention relates to a tag axle assembly attached to the rear end of a truck. Specifically, the invention relates to a tag axle assembly attached to the rear end of the frame of a truck whereby the wheels of the tag axle may be moved between a ground engaging position for increasing the load carrying capacity of the truck, and a non-ground engaging position in which the axle and wheels are positioned adjacent the truck in such a manner that the auxiliary tag axle assembly does not impede the free movement of truck components.
2. Background Information
Transit concrete mixing trucks typically include a frame, a cab for the operator, and a rotatable drum mounted to the frame behind the cab for containing concrete ingredients. Such trucks further typically include a steering axle near the cab, and rear drive axles are often provided in a dual axle arrangement. Trucks of this type often include additional auxiliary axles to assist the drive and steering axles in supporting the truck load. Auxiliary axles of this type may take the form of a pusher axle located ahead of the drive wheels or a tag axle mounted to the rear of the truck. Additionally, it is common for auxiliary axles to be movable between a ground engaging and non-ground engaging position.
In the ground engaging position, the auxiliary axle assists the drive and steering wheels in supporting the truck when it is partially or fully loaded. In the non-ground engaging, or raised position, the axle is moved upwardly and inwardly toward the truck frame such that the wheels associated with the auxiliary axle do not engage the ground, and the auxiliary axle does not assist the drive and steering axles in supporting the truck load. While pusher and tag axles perform primarily the same function, tag axles offer an additional advantage in that the axle may be positioned behind the truck frame, substantially increasing the distance over which the load is supported thereby permitting the truck to carry larger loads.
More specifically, tag axle assemblies are often attached to the rear end of a truck such that when the tag axle is in the ground engaging position, it extends rearwardly of the frame by a significant distance to increase the area overwhich the load is distributed to the road surface. However, the tag axle must also be conveniently stowed in a non-ground engaging position such that the tag axle assembly does not interfere with the movement of the truck components, and specifically the concrete delivery chutes.
Tag axle assemblies of this type can be an impediment when the vehicle reaches the construction site and must travel over rough, uneven and sometimes muddy or soft terrain. Additionally, tag axle assemblies may include steerable or non-steerable axles. Steerable axles, sometimes referred to as self-steering axles, allow the tire assembly to follow the truck as it negotiates turns by rotating relative to the axle to prevent tire scuffing and increase truck maneuverability. However, steerable axles generally have a caster angle such that the axle is self-steering only when the truck is moving in the forward direction. When the truck moves in reverse, for example, to move the concrete delivery chute adjacent a concrete form, the axle will not steer and the tires will scuff along the ground. Such scuff will damage the tires, and possibly the tag axle assembly. As such, the tag axle must be moved to a non-ground engaging position when the vehicle moves in reverse if the tag axle assembly includes a steerable axle.
Thus, a very important element of any tag axle assembly design is the above described ability to move between ground engaging and non-ground engaging positions. Additionally, when the assembly is in the non-ground engaging position, the assembly should have as much ground clearance as possible to allow the truck to negotiate obstructions at the construction site. Further, the tag axle assembly can not impede the free movement of the concrete delivery chutes.
Specifically, there is a compromise between the desire to elevate the tag axle assembly as high as possible to avoid obstructions and allow the truck to get close to concrete forms and the desire to not raise the tag axle assembly so high that it interferes with the free movement of concrete delivery chutes, etc. Additionally, it is important that the axle is positively secured in the raised position to prevent the tag axle from inadvertently dropping into ground engagement possibly causing property damage or bodily injury.
One drawback of air spring tag axle suspensions is that an air spring is essentially an air inflated bag and requires auxiliary stabilization. An air suspended axle must have separate independent mechanical location and attitude controls and stabilized components or it will not function. Absent stabilization, the air spring will extend to its maximum length or width in the direction of least resistance.
A significant number of air spring suspensions have been developed which to a greater or lesser extent, control axle location and attitude. A number of suspensions that have been developed are roll rigid, others are roll flexible, each generally being designed for specific application. The most common roll rigid configuration is the trailing beam type suspension, most of which use the axle as a torsion bar to provide roll rigidity.
Another type of suspension which has been developed is the parallelogram suspension which is not inherently roll rigid, and does not inherently provide lateral stiffness. While parallelogram suspensions provide the above problems not found in trailing arm type suspensions, they provide the further advantage in that the suspension has an inherent ability to maintain a constant caster angle for caster steering axles which are often utilized in auxiliary tag axle assemblies for trucks.
The parallelogram suspension inherently provides the above advantage, and also locates the axle relative to the longitudinal axis of the vehicle by controlling the forward and rearward motion of the axle relative to the frame. Moreover, a parallelogram suspension also controls the path which the air spring follows as it operates to take up irregularities in the road surface while simultaneously assuring the tire assembly remains at a constant caster angle at all positions in the vertical path of the tire assemblies.
The design of reliable and economical tag axles has become increasingly important in recent years as many governmental agencies enforce road restrictions limiting the amount of weight any vehicle can carry while traversing roads and highways within their jurisdiction. Federal weight laws have a specific formula involving axles, the spacing of axles, and the length of the truck wheel base. Greater loads may, therefore, legally be transported by increasing the number of load bearing axles and/or wheel base to help support the vehicle.
Weight restrictions also have been expressed as a function of the distance between axles such that increasing the distance between axles assures that the truck weight is distributed over a greater road surface area. Many restrictions of this type can be met by increasing the distance between the rearmost axle and the steering axle of the truck by supplying an auxiliary axle assembly which can serve as a rear axle.
Various companies, including McNeilus Truck and Manufacturing, Inc., for example, have patented devices to help vehicles meet load limit restrictions. Specifically, U.S. Pat. Nos. 4,492,389 and 4,762,421 have issued on tag axle assemblies. However, the prior art does not provide the extension available in the ground engaging position of the present invention, while simultaneously providing an extremely compact stowable tag axle assembly when in the non-ground engaging position such that the concrete discharge chutes may be easily maneuvered around the tag axle assembly in both the ground engaging and non-ground engaging assembly. Moreover, none of the existing designs provide the safety provided by the present invention.
Therefore, the need exists for a tag axle assembly movable between a ground engaging and a non-ground engaging position. Moreover, a need exists for such a tag axle assembly in which the tag axle significantly extends the distance between the front steering axle, and the rearmost ground engaging axle, and in which the tag axle assembly is easily stowed on the truck to permit free movement of truck components without interference from the auxiliary tag axle assembly.